Electric servo-assisted steering system for motor vehicles

ABSTRACT

An electric power assisted steering system for motor vehicles having a worm gear mechanism is operatively connected to a drive shaft of an electric drive motor. A radial load is applied to the worm by a prestressing device, the prestressing device being configured as a hydraulic pressure device. It is possible to connect the drive shaft to the worm by a coupling, and the worm is mounted in this region by a pivotable bearing.

The invention relates to an electric power assisted steering system formotor vehicles having a worm gear mechanism in accordance with theprecharacterizing clause of claim 1.

An electric power assisted steering system of the generic type for motorvehicles is disclosed in DE 199 44 133 A1.

The document of the generic type reveals an electric power assistedsteering system having an input shaft which is operatively connected toa steering handwheel and serves for transmitting a torque which isrequired to steer steerable wheels. An output element is operativelyconnected to the steerable wheels and has power assistance applied to itby an electric drive motor. A stepdown gear mechanism configured as aworm gear mechanism is connected to the electric drive motor and theoutput element. A prestressing device, which prestresses the wormradially or subjects it to a radial load, is provided in order toeliminate the noises produced as a result of the play in the worm gearmechanism. The play in the worm gear mechanism is eliminated by theslight radial prestressing between the worm and the worm gear. When theradial prestressing is configured, it must be adapted such that thefrictional forces produced do not cause the worm gear mechanism to jam.

Although the tooth-flank play is improved as a result of the document ofthe generic type, the forces which occur between the worm and the worngear in the operating state cannot be completely compensated for. Themaximum prestressing effect of the prestressing device is limited, asthe worm gear mechanism would otherwise jam. However, the operatingforces which occur are far above the maximum prestressing force, whichis usually 15 to 20 N.

In relation to the further prior art, reference is also made to DE 19822 478 A1. Provision is made here for an elastic body to be arrangedsuch that, if a tooth surface of the worm are placed in contact with oneanother with a tooth surface of the worm gear, a rotary shaft is moved,with deformation of the elastic body.

In similar fashion to the document of the generic type, it is also notpossible to compensate for the operating forces of the worm gearmechanism by this means, and it is thus not possible to advantageouslyreduce noise.

Further disadvantages, which are eliminated neither by the solutionsdisclosed in the prior art nor by the two cited documents, comprise thewear increasing over the service life and thus the tooth-flank play andhence the rattling and vibration noises increasing further. Wearadjustment is not possible.

Disadvantageously, the known solutions are also not capable of settingthemselves automatically. A significant simplification of the work andtherefore reduction in costs would result if the worm were capable ofsetting itself automatically in relation to the worm gear, in particularwhen new.

Moreover, the known solution has the disadvantage that temperaturefluctuations and also fluctuations in the relative air humidity, whichhave varying effects on the worm gear mechanism on account of varyingmaterial coefficients of expansion and coefficients of water absorption,cannot be compensated for. This can likewise lead to rattling andvibration noises, and also to the worn gear mechanism distorting andjamming. The influence of temperature fluctuations is also a problembecause the crown gear used is frequently formed from plastic and theplastic used for this purpose has a relatively high coefficient ofthermal expansion. Generally, such plastics are unreinforced, as theyare the most suitable for this purpose, but they are subject torelatively large expansion in the event of severe temperaturefluctuations. Said expansion of the plastic can result in the worm gearmechanism distorting. This also applies analogously when water isabsorbed from the air and the resulting plastic swelling occurs.

The present invention is therefore based on the object of eliminatingthe abovementioned disadvantages of the prior art, in particular ofimproving an electric power assisted steering system for motor vehicleshaving a worm gear mechanism to such an extent that noise is reduced toa minimum, automatic setting when new and wear adjustment are possible,and, moreover, temperature fluctuations as well as fluctuations in therelative air humidity can be compensated for.

This object is achieved according to the invention by the characterizingfeatures of claim 1.

The various tasks, firstly adjustment of a prestressing force andsecondly absorption of the operating force, are separated by virtue ofthe fact that the prestressing device is configured as a hydraulicpressure device. It is possible to hold the worm in engagement with theworm gear without play by means of a prestressing force, while theoperating forces are compensated for by the hydraulic damping. It isthus possible to select a small prestressing force and at the same timeachieve, by means of the hydraulic damping, such a damping effect thathigh operating forces can be absorbed without problems.

The hydraulic pressure device affords a further advantage duringinstallation of the steering system. The hydraulic pressure device setsitself automatically, with the result that it is not necessary to setthe tooth-flank play of the worm gear mechanism. This makes it possibleto install the apparatus according to the invention separately.

The worm gear mechanism can be set optimally and automatically by virtueof the fact that it is possible to connect the worm to the drive shaftby means of a coupling and a pivotable bearing arranged in this region.The wear which occurs over the service life can thus be compensated forautomatically, so that the tooth-flank play is not increased andrattling and vibration noises are avoided. Fluctuations, which resultfrom the relative air humidity or as a function of temperature from thevarious coefficients of expansion of the materials, can likewise becompensated for automatically by the automatic adjustment, brought aboutby the pivotable bearing and the hydraulic pressure device. It is thuspossible firstly to avoid rattling and vibration noises and secondly toreliably prevent the worm gear mechanism distorting or jamming as aresult of certain materials expanding excessively.

The solution according to the invention advantageously ensures that thetoothing of the worm gear mechanism engages without play and the highoperating forces which occur are compensated for.

Furthermore, it may be provided according to the invention for the wormto be prestressed by a prestressing spring of the hydraulic pressuredevice.

As has emerged in tests, the various tasks can be separated particularlyeffectively and inexpensively by the prestressing spring. Tooth-flankplay can be suppressed by suitably adjusting the prestressing spring,while at the same time the high operating forces which occur arecompensated for by the hydraulic component of the hydraulic pressuredevice. The toothing of the worm is thus pressed without play intotoothing engagement with the worm gear by means of the prestressingspring.

It is advantageous for the pivotable bearing to be configured as a fourpoint bearing with a pivot ring.

Firstly, this refinement makes larger angles for adjusting the wormpossible, and secondly a lack of axial play is nevertheless ensured. Thenoise problems are suppressed by the fact that there is no axial play inthe bearings. The advantage of conventional four point bearings, whichis that they have very small axial play and thus no noise occurs as aresult of axial impact (i.e. noise in the form of ball impact when thedirection of the axial forces changes), can be utilized by the pivotring. The characteristic feature of conventional four point bearings,namely that they only permit very slight oblique positions of the shaft,in this case of the worm, is advantageously overcome by arranging thefour point bearing in the pivot ring. It is thus possible to mount theworm axially with very little play and nevertheless to achieve a largeoblique position without increased coefficients of friction and noiseoccurring.

This is advantageously possible by grinding the outer ring of thebearing to be spherical or rounded and by installing a pivot ring, whichallows the four point bearing to pivot, on the spherical outer ring ofthe four point bearing.

Advantageous developments and refinements of the invention emerge fromthe further subclaims and from the exemplary embodiments described belowin basic form using the drawing, in which:

FIG. 1 shows a section through a worm gear mechanism which is connectedto a drive shaft of an electric drive motor;

FIG. 2 shows a first embodiment of a hydraulic pressure device;

FIG. 3 shows a second embodiment of a hydraulic pressure device;

FIG. 4 shows a section through a coupling having two sheet metalmoldings injection molded into an elastomer; and

FIG. 5 shows a plan view of the sheet metal moldings in accordance withFIG. 4.

Electric power assisted steering systems for motor vehicles having aworm gear mechanism are sufficiently known, for example from DE 199 44133 A1, for which reason only the features essential to the inventionare discussed in greater detail in the following text.

FIG. 1 shows a detail of an electric power assisted steering system formotor vehicles having a worm gear mechanism 1 arranged in a housing 2.The worm gear mechanism 1 has a worm 3 and a worm gear 4. The worm 3 ismounted in a pivotable bearing 5. In the present exemplary embodiment,the pivotable bearing 5 is formed from a four point bearing 6 which isarranged in a pivot ring 7. The worm 3 is connected to a drive shaft 9,which is operatively connected to a drive motor 10 (not shown in greaterdetail), by means of a coupling 8 (shown in greater detail in FIG. 4 andFIG. 5).

The drive shaft 9 is mounted in a bearing 11 in the usual fashion.

As can likewise be seen from FIG. 1, the worm 3 is acted on radially bya hydraulic pressure device 12. By means of the hydraulic pressuredevice 12, the worm 3 and the worm gear 4 are made to engage withoutplay and the operating forces occurring are compensated for. Here, theprestressing force is provided by a prestressing spring 13. A hydraulicdevice 14, which is known in principle, of the hydraulic pressure device12 is provided to compensate for the operating forces occurring. Twoadvantageous hydraulic pressure devices 12 can be seen in FIGS. 2 and 3.

A pressure piece 15 of the hydraulic pressure device 12 acts on abearing 16, arranged in this region, of the worm 3 and thus influencesthe play between the worm 3 and the worm gear 4. In order that thebearing 16 or the worm 3 can be moved correspondingly radially by thehydraulic pressure device 12 or the pressure piece 15, it is feasible toarrange said bearing 16 or worm 3 in a slot or a groove which is easilyconceivable and therefore not shown in greater detail. Moreover, the useof suitable bearings is possible, for example of an oscillating bearing.

A nonreturn valve 17 having an orifice bore 18 can be seen in FIG. 2,said nonreturn valve 17 taking over a function known in principle in thehydraulic device 14 and thus permitting a flow of oil to flow away froman oil space 19 in a manner throttled only by the orifice bore 18, ifsaid oil space 19 is subjected to a corresponding load by the pressurepiece 15 by means of a plunger 20. The hydraulic pressure device 12shown in FIG. 2 is in principle of a known construction, for whichreason only the essential features have been discussed. In order tocompensate for production related deviations in roundness, the hydraulicpressure unit 12 has a compression spring 21 which compensates for thehighly dynamic components of the toothing play between the worm 3 andthe worm gear 4.

It is disadvantageous, however, in the case of the hydraulic pressuredevice 12 shown in FIG. 2 that, if the oil space 19 is subjected to acorresponding pressure, the resulting high pressure acts on the sealingring 22 and thus high demands are made on the latter with regard to thesealing effect. The hydraulic pressure device 12 shown in FIG. 3 offersan alternative advantageous solution to this.

The hydraulic pressure device 12 shown in FIG. 3 has a nonreturn valve17 which separates the oil space 19, to which high pressure can beapplied, from a low pressure oil space 24 by means of a ball 23. Themethod of operation here is analogous to the hydraulic pressure device12, already described in FIG. 2, and is known in principle. Acorresponding movement of the pressure piece 15 builds up a highpressure in the oil space 19 counter to the force of the prestressingspring 13 and through the nonreturn valve 17 which high pressure canonly dissipate through a gap 25 in a highly throttled manner. As aresult, the high operating forces between the worm 3 and the worm gear 4are compensated for. The gap 25 can measure, for example, 3μ and isarranged between an inner wall of the plunger 20 and an outer wall ofthe housing of the nonreturn valve 17. As a result of the configurationshown in FIG. 3, the seal 22 is loaded only with the pressure in the lowpressure oil space 24.

As a result of the nonreturn valve 17, or the orifice bore 18 (FIG. 2)or the ball 23 (FIG. 3), the hydraulic device 14 can compensate, over adefined time constant, for the thermal expansion and the waterabsorption of the worm 3 by it being possible for the plunger 20 toyield in a defined manner.

An advantageous coupling of the worm 3 to the drive shaft 9 is shown inFIGS. 4 and 5. The coupling 8 is formed from two metal or sheet metalmoldings 26 which have been vulcanized into an elastomer 27. The sheetmetal moldings 26 are shaped by drivers such that they still afford anemergency running property if the elastomer 27 is destroyed. Thecoupling to the worm 3 or the drive shaft 9 can be effected byrectangular drivers on the respective shafts 3, 9.

As has been shown in tests, a coupling 8 of this type can be realized ina particularly simple and cost saving manner. As shown in FIG. 5, thesheet metal moldings 26 are advantageously laid on top of one another inthe form of a cross and subsequently vulcanized into the elastomer. Ithas proven particularly suitable to arrange the two sheet metal moldings26 offset by 90°. It is naturally also possible to provide more sheetmetal moldings 26 in a different arrangement.

LIST OF DESIGNATIONS

-   1 Worm gear mechanism-   2 Housing-   3 Worn-   4 Worm gear-   5 Pivotable bearing-   6 Four point bearing-   7 Pivot ring-   8 Coupling-   9 Drive shaft-   10 Drive motor-   11 Bearing (drive shaft)-   12 Hydraulic pressure device-   13 Prestressing spring-   14 Hydraulic device-   15 Pressure piece-   16 Bearing (worm)-   17 Nonreturn valve-   18 Orifice bore-   19 Oil space-   20 Plunger-   21 Compression spring-   22 Sealing ring-   23 Ball-   24 Low pressure oil space-   25 Gap-   26 Sheet metal moldings-   27 Elastomer

1. An electric power assisted steering system for a motor vehicle,comprising: an electric drive motor; a worm gear mechanism operativelyconnected to a drive shaft of the electric drive motor; a prestressingdevice arranged to apply a radial load to a worm and configured as ahydraulic pressure device; a coupling configured to connect the driveshaft to the worm; and a pivotable bearing, the worm mounted by thepivotable bearing, wherein the hydraulic pressure device includes anonreturn valve arranged to damp operating forces of the worm gearmechanism acting in a radial direction.
 2. The electric power assistedsteering system according to claim 1, wherein the hydraulic pressuredevice includes a compression spring arranged to absorb highly dynamicforces from the worm gear mechanism.
 3. The electric power assistedsteering system according to claim 1, wherein the hydraulic pressuredevice includes a prestressing spring arranged to prestress the worm. 4.The electric power assisted steering system according to claim 1,wherein the prestressing device is configured to compensate an operatingforce.
 5. The electric power assisted steering system according to claim1, wherein the pivotable bearing includes a pivot ring.
 6. An electricpower assisted steering system for a motor vehicle, comprising: anelectric drive motor; a worm gear mechanism operatively connected to adrive shaft of the electric drive motor; a prestressing device arrangedto apply a radial load to a worm and configured as a hydraulic pressuredevice; a coupling configured to connect the drive shaft to the worm;and a pivotable bearing, the worm mounted by the pivotable bearing,wherein the pivotable bearing is configured as a four point bearinghaving a pivot ring.